The invention relates to a container liner, preferably for use in flexible bulk goods containers.
Flexible bulk goods containers are used for the packaging, transporting, wrapping and stockpiling of different bulk goods materials, in each case in amounts of 500 to 2000 kg, and are also used in areas, in which there is a danger of explosion. This danger of explosion can arise, for example, when filling a bulk goods container (FIBC) with materials capable of producing dust explosions. When such an FIBC is emptied, there is moreover not only the danger of a dust explosion, but also the danger of a vapor or gas explosion, if the FBIC is emptied into a container, in which there already is a flammable liquid. To avoid the danger of ignition resulting from electrostatic charges, bulk goods containers must be used, which are electrostatically conductive on the inside and the outside and grounded, in all areas, in which there is a danger of explosion due to flammable gases, vapors or dust.
Therefore, when designing electrostatically conductive bulk goods containers, the risks of a brush discharge, in the case of gases and vapors with a low minimum ignition energy, as well as the risks of sliding-style brush discharge, which is attributed particularly to charged double layers, must be taken into consideration. Likewise, however, it is also necessary to make certain that the bulk goods container has strength properties, which permit stockpiling of bulk goods up to 2 tons, and is sufficiently tight, so that dust does not emerge through the woven fabric.
Usually, flexible bulk goods containers are produced from flexible polypropylene tape. By weaving in electrically conductive tapes in the warp and filling directions, the basic fabric of the container is divided into electrostatically separate area elements not larger than 10.sup.4 mm.sup.2, so that brush discharges, capable of causing an ignition, cannot start out from the charged surfaces of the insulating material.
Since the flexible bulk goods containers of woven tapes inherently do not prevent the passage of dust, they are provided on the inside of the container with a coating preferably of polypropylene.
For certain high-grade chemical products and sensitive foods, it is moreover necessary to make a barrier layer available, which has a low permeability for water vapor, nitrogen, oxygen, carbon dioxide or aromas contained in the food. Such a barrier layer may be realized in the form of a container liner, which is inserted in the flexible transport containers as an internal container. As a result, the liner itself may have comparatively lower strength properties. However, in its interaction with the external transport container, it must fulfill the requirements for preventing electrostatic charges.
For this purpose, there are protection classes, which the containers must fulfill and which can be looked up, for example, at the professional association of the chemical industry. Containers, which are assigned to the protection class B (only danger of a dust explosion), may contain a chargeable container material; however, sliding-style brush discharges must not arise. These are avoided reliably if the electric breakdown voltage, measured over the whole of the wall structure (container with or without an inner sack) does not exceed 4 kV.
Containers of protection class C (additional explosion danger due to gases and vapors) must be capable of dissipating electrostatic charges, that is, the leakage resistance must be less than 10.sup.8.OMEGA. at each place of the inside and outside of the container (including the lamination and/or the inner sack) and the container must be grounded.
In order to fulfill these requirements with respect to the leakproofness of such a container on the one hand and with respect to the ability of the wall material to dissipate electrostatic charges on the other, it was proposed (DE 43 09 832 A1) that an electrically insulating layer on a conductive support be perforated. By these means, the insulation of the conductive support is perforated pointwise and the danger of discharges capable of igniting is prevented (protection class C). This perforation is produced by electric breakdown of the insulation.
Such coated wall material, the charge on which can be dissipated electrostatically, does not have adequate barrier properties for certain chemical products and foods. Moreover, it is necessary to produce the walls of such containers with the help of sewing machines. Needle-insertion holes decrease leakproofness considerably and permit penetration by water during outside storage. It is a further problem that adequate cleanliness for pharmaceutical products cannot be attained by the manufacturing step.
In order to attain adequate barrier properties, outside storage capabilities and/or adequate cleanliness for containers capable of dissipating electrostatic charges, liners are also already being used at the present time. However, they do not fulfill or do not fulfill reliably the requirements of the above-mentioned classes B and C, such as liners with antistatic active ingredient concentrates (such as ethoxylated alkylamine). The antistatic effect is achieved owing to the fact that the active ingredients migrate to the surface of the liners and bind moisture. A surface resistance of 10.sup.9.OMEGA. to 10.sup.11.OMEGA. is achieved. The effect depends on the relative humidity of the air in the surroundings of the liner. Physiological safety is not an established fact. The effect is time-limited (not more than 18 months).
Polymeric monoflms, into which conductive carbon black compounds are incorporated, are also used on the market as liners. Carbon black compounds are very expensive. Moreover, they have the disadvantage that they are classified as carcinogenic and that the sealed seam strength of these films is very slight. Because of these disadvantages, the films are also offered in co-extruded form, usually with a layer about 75 .mu.m thick, with conductive carbon black compounds and a layer 25 .mu.m thick without conductive carbon black compounds. The 25 .mu.m thick layer is to protect the contents of the container against the negative properties of the conductive carbon black compounds. On the other hand, the 25 .mu.m layer is intended to ensure that the breakdown voltage of 4 kV is not exceeded. On the basis of the state of the art, it is not possible to reduce the 25 .mu.m layer thickness further, in order to attain a breakdown voltage, which is clearly less than 4 kV. It must also be expected that the thickness of the layer is not uniform and that therefore the desired breakdown voltage is exceeded in parts. Moreover, such a wall material cannot fulfill the barrier properties addressed above, which are required for certain chemical products and foods.